Low-bit-rate speech coding schemes have been proposed wherein the flow of speech signal samples, originally coded at a relatively high bit-rate, is split into consecutive blocks of samples, each block being then re-coded at a lower bit rate using so called Vector Quantizing (VQ) techniques. VQ techniques include for instance so called Pulse-Excited (RPE or MPE) as well as Code Excited Coding. More efficient coding has also been achieved by combining Vector Quantizing with Linear Predictive Coding (LPC) wherein bandwidth compression is performed over the original signal prior to performing the VQ operations. To that end, the speech signal is first filtered through a vocal tract modeling filter. Said filter (Short Term-Predictive (STP) filter) is designed to be a time invariant, all-pole recursive digital filter, over a short time segment (typically 10 to 30 ms, corresponding to one or several blocks of samples). This supposes first an LPC analysis over said short time segment to derive the filter coefficients, i.e. prediction coefficients, characterizing the vocal tract transfer function. Then the time-variant character of speech is handled by a succession of such filters with different parameters, i.e. by dynamically varying the filter coefficients.
Filter coefficients derivation operation obviously mean processing delay adding to the otherwise coding delay due to further processing including VQ operations. This leads to total delay in the order of 25 to 80 ms depending on the type of signal processor being used.
Such a delay is not compatible with the specifications of speech coders to be used in the public switched network without echo cancellation. More particularly, no known technique fits to a low bit rate (e.g. 16 kbps) which would provide a low delay, while still keeping high coding speech quality, with an acceptable coder complexity.